Distraction instrument with fins for maintaining insertion location

ABSTRACT

A tip for a medical device that distracts two opposing vertebral bodies. The tip has a hub, a boss, and a first longitudinal fin. The boss is attached the hub and has a body, a tapered portion, and a vertex. The first fin is attached to the boss, and projects from at least a portion of the boss&#39;s external surface. The first fin then extends over at least a portion of the length of the body and substantially extends over the length of the tapered portion.

[0001] The present invention relates to an apparatus for separatingvertebral bodies to perform surgery or install implants in the discspace between vertebral bodies, and more specifically relates to anapparatus with one or more fins that extend from a substantial part ofthe tapered end and the body of the apparatus in order to maintain itsinsertion location between the vertebrae.

BACKGROUND

[0002] Distraction is a common surgical procedure to operate on thespine. It is used to prepare a spine for fusing two opposing (adjacent)vertebrae or to install an implant in the disc space between twoopposing vertebrae. Distraction can be performed anteriorly (from thefront of the patient) or posteriorly (from the back). But regardless ofthe approach, the procedure generally begins by first exposing thatportion of the spine, which the surgeon has determined to requirerepair, and then removing all or part of the damaged spinal disc betweenthe two opposing vertebrae in question.

[0003] After removing the spinal disc, the surgeon then typicallyinserts what the layperson may describe as one or more wedges betweenthe two vertebrae. Each wedge is mounted on the end of a shaft andsuccessively taller wedges are inserted between the two vertebrae untilthe surgeon obtains the desired separation (“distraction”) between them.Once the desired separation is obtained, the surgeon then slides whatthe layperson may describe as a special tube over the shaft while thelast wedge is still located between the vertebrae. This special tube isoften referred to as a “guide sleeve”. The guide sleeve is generallytaller than the wedge that separates the vertebrae. Hence, the distalend (the end toward the patient) of the guide sleeve butts against theoutside of the top and bottom vertebrae. Typically, the guide sleeve hasa top spike and bottom spike that extend from its distal end, and thesespikes are respectively placed in contact with the top and bottomvertebrae. Further, the end of the guide sleeve may have lateralextensions to extend into the disc space between the top and bottomvertebrae and around each side of the wedge. The spikes are then driveninto the vertebrae by striking the proximal end of the outside sleevedistractor with a hammer or some other impact device. With the spikesplaced into the vertebrae and the lateral extensions slid in the discspace, the wedge is no longer necessary to hold the vertebrae apart. Thespikes and lateral extensions now serve that function, and the wedge canbe pulled out from the disc space through the inside of the guidesleeve. With the wedge removed, the surgeon is then free to work throughthe inside of this special tube to prepare the disc space for fusion orinstall implants.

[0004] With the foregoing explanation in mind, some nomenclature isimportant to fully understand the following specification. If thewedge/tip used to separate the vertebrae is approximately as tall as itis wide, the operation is called a “single barrel” procedure.Alternatively, if the wedge/tip is approximately twice as Side as it istall, the operation is called a “double barrel” procedure. Thisterminology has largely risen for metaphoric reasons. The cross-sectionof a guide sleeve used in a double-barrel procedure resembles thecross-section of a double-barrel shotgun, and the cross-section of theguide sleeve used in a single-barrel procedure resembles thecross-section of a single-barrel shotgun.

[0005] The insertion location of the wedges between the vertebrae is ofcritical importance in most any surgery involving vertebral distraction.The wedge/tip determines the placement of the guide sleeve, which inturn commonly determines the exact placement of an implant or fusionmaterial within the disc space. Hence, prior art devices, such as thatpresented in U.S. Pat. No. 5,484,437 to Michelson seek to limitinadvertent migration of the wedge by incorporating sharp pegs into thetop and bottom sides of the wedge. The pegs in that design, however, donot contact vertebral tissue until the distractor/wedge has all butfully separated the vertebrae in question. As a result, that prior artdesign may allow the distractor/wedge to migrate during insertion intothe vertebral space, before the sharp pegs have the opportunity tofunction.

[0006] What is needed is a medical device that can immediately secureits position within the disc space between two vertebral bodies, oncethe surgeon places the device in contact with the spine. The followingdevices address this need.

SUMMARY OF THE INVENTION

[0007] In one aspect, this invention is a tip for a medical device thatdistracts two opposing vertebral bodies. The tip has (1) a hub, (2) aboss, and (3) a first longitudinal fin. The boss is attached to the huband has a body, a tapered portion, and a vertex. The first longitudinalfin is attached to the boss, and projects from at least a portion of theboss's external surface. But in addition, the first longitudinal finalso extends over at least a portion of the length of the body andsubstantially extends over the length of the tapered portion of theboss.

[0008] In another aspect, this invention is a tip for a medical devicethat distracts two opposing vertebral bodies. The tip has (1) a hub, (2)a boss, (3) a first longitudinal fin, and (4) a second longitudinal fin.The boss is attached to the hub and has a body, a tapered portion, and avertex. The first and second longitudinal fins are attached to the boss,and project from at least a portion of the boss's external surface. Butin addition, the longitudinal fins also extend over at least a portionof the length of the body and substantially extend over the length ofthe tapered portion of the boss.

[0009] As used in this specification, the term “hub” is a part of thisinvention that holds the boss. The hub may have various shapes such as awedge, a tube, a rectangle, a square, a parabola, or a dome.

[0010] As used in this specification, the term “boss” is a part of thisinvention that holds the longitudinal fins. The boss may have variousshapes such as a wedge, a tube, a rectangle, a square, a parabola, or adome.

[0011] As used in this specification, the term “substantially extendingover the length of said tapered portion”, which is used to describe theplacement of a longitudinal fin on the present invention, means that thelongitudinal fin distally extends over the tapered portion for no lessthan about 40 percent of the total length of the tapered portion,measured over the external surface of the tapered portion, between thetapered portion's proximal and distal ends.

DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a perspective view of a tip for a medical device todistract two opposing vertebral bodies according to one embodiment ofthe present invention.

[0013]FIG. 2 is a plan view of a tip for a medical device to distracttwo opposing vertebral bodies according to one embodiment of the presentinvention.

[0014]FIG. 3 is a side elevation view of a tip for a medical device todistract two opposing vertebral bodies according to one embodiment ofthe present invention.

[0015]FIG. 4 is a side elevation view of a longitudinal fin, which maybe used to practice the present invention.

[0016]FIG. 5 is a side elevation view of a medical device to distracttwo opposing vertebral bodies according to another embodiment of thepresent invention.

[0017]FIG. 6 is a plan view of a medical device to distract two opposingvertebral bodies according to another embodiment of the presentinvention.

[0018] FIGS. 7-10 are respectively a perspective view, a plan view, anend view, and a cross-sectional view of a boss, which may be used topractice the present invention.

[0019] FIGS. 11-12 are respectively a side elevation view and an endview of a shaft, which may used to practice the present invention.

[0020] FIGS. 13-15 are respectively a perspective view, a plan view, andan end view of an aft guide, which may be used to practice the presentinvention.

[0021]FIGS. 16 and 17 are respectively a front perspective view and arear perspective of a hub, which may be used to practice the presentinvention.

[0022] FIGS. 18-20 are respectively a plan view, side cross-sectionview, and an end view of a hub, which may be used to practice thepresent invention.

[0023]FIG. 21 is a side elevation view of one embodiment of the presentinvention being inserted between two opposing vertebral bodies.

[0024] FIGS. 22-24 are respectively a top, side, and bottom view of yetanother embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, and that suchalterations and further modifications in the illustrated device and thatsuch further applications of the principles of the invention asillustrated therein are contemplated as would normally occur to oneskilled in the art to which the invention pertains.

[0026] Referring now to FIGS. 1-3, there is shown a tip 30 for a medicaldevice that distracts two opposing vertebral bodies according to oneembodiment of the present invention. Tip 30 includes a hub 31, a boss34, and a longitudinal fin 35. Hub 31 has a proximal end 32, a distalend 33, and means for connecting proximal end 32 to the distal end of amedical device (not shown). In this embodiment, the means for connectingproximal end 32 to a medical device are female screw threads 36. As iscommon in the art, these threads are created by first drilling hole 44into hub 31, followed by tapping threads 36 into the walls of hole 44.However besides female screw threads, other connecting means arecontemplated to practice the present invention such as male screwthreads, pinning, or welding. The end of hub 31 is also shown with bands56 and 40. These bands are preferably colored to designate a tip of aparticular size.

[0027] Boss 34 has a proximal end 37, a distal end 38, and a body 50located between ends 37 and 38. Proximal end 37 is fixedly secured tothe distal end 33 of hub 31 in any appropriate manner such as welding,however, both hub 31 and boss 34 are more preferably manufactured bydirectly machining hub 31 and boss 34 from a common piece of material.Nevertheless, hub 31 is also preferably larger than boss 34 so as toform shoulder 55 between hub 31 and boss 34. Body 50 has a length 60(FIG. 3), and preferably, length 60 is such that tip 30 will not overlypenetrate the disc space when shoulder 55 butts against the outside ofthe vertebrae. The distal end 38 of boss 34 is closed and preferablygets thinner at tapered portion 62 over a surface length 61 to a vertex39. Similar to hub 31, boss 34 is cut with bands 41, 42, and 43;however, unlike the bands in the hub, these bands in boss 34 serve toengage tissue when the tip is in use. Finally, boss 34 is also cut withslot 45 to hold longitudinal fin 35, and preferably, slot 45 is cutcompletely through boss 34.

[0028] Now referring to FIGS. 1-4, longitudinal fin 35 has a proximalend 46 and a distal end 47 that preferably extends from boss 34 atvertex 39 and continues to extend from boss 34 for the entire surfacelength 61 of tapered portion 62. However, it is also contemplated thatlongitudinal fin 35 does not extend from surface length 61 for itsentire dimension, that is from its distal end at vertex 39 to itsproximal end 59. In this regard, it is also contemplated thatlongitudinal fin 35 may also be coterminous with the tapered portion (orsimply does not extend from the surface of the tapered portion at thislocation) for up to about the 60 percent of surface length 61. And morepreferably does not so extend for up to about the first, distal 60percent of surface length 61. In other words, the fin 35 will extendfrom tapered portion 62 for about 40 percent or more of length 61 andleave a remaining 60 percent or less without a fin. But again, morepreferably longitudinal fin 35 will extend from tapered portion 62 forabout the first, proximal 40 percent or more of length 61 and leave aremaining distal 60 percent or less without a fin. An example of a 60percent coterminous profile is depicted by the dashed line 64 in FIG. 4with the dashed line representing the outside limits of boss 34 thatreside next to fin 35. Dashed lines 65 and 66 respectively represent afin with about a 20 percent and about a 10 percent coterminous profile.

[0029] Longitudinal fin 35 is preferably mechanically held in slot 45 bypins 54 (FIG. 3) or by a weld (not shown). The proximal end 46 oflongitudinal fin 35 is preferably straight to ride against the bottom 48of slot 45. The distal end 47, however, preferably terminates at asecond vertex 49. Then as shown, longitudinal fin 35 preferablycontinues from vertex 49 to project laterally around the tapered portion62 of distal end 38 and down at least a portion of body 50. Moreover,although longitudinal fin 35 is shown in FIG. 3 wrapping around both thetop side 52 and the bottom side 53 of boss 34, it is also contemplatedthat longitudinal fin 35 may project only from one of these sides, notboth. And still further, though fin 35 is shown as a separate piece, itis further contemplated that fin 35 and boss 34 are cast or machinedfrom a common piece of material.

[0030] The tip shown in FIGS. 1-3 would typically be used in asingle-barrel procedure and is further of a design that is capable ofbeing removably attached to a medical device. In this regard, the readershould take note the present invention may take other forms. Forexample, this single barrel design could be permanently secured to ashaft as part of a larger medical device that is used to distract twoopposing vertebrae, or this tip could be widened for use in adouble-barrel procedure. For the convenience of the reader, anembodiment incorporating both of these options follows.

[0031] Referring now to FIGS. 5 and 6, there is shown a medical device80 that distracts two opposing vertebral bodies according to anotherembodiment of the present invention. Medical device 80 includes a shaft81, a hub 82, a boss 92, a first longitudinal fin 83, and a secondlongitudinal fin 84. Hub 82 has a proximal end 85, a distal end 86, andmeans for connecting proximal end 85 of hub 82 to the distal end 87 ofshaft 81. In this embodiment, the means for connecting proximal end 85to shaft 81 is a pin and a weld. As is common in the art, such a jointis created by first drilling a hole 88 into hub 82 that is of slightlylarger diameter than shaft 81. Shaft 81 is then inserted into hole 88,which is followed by drilling a much smaller hole 89 into the side ofhub 82 and shaft 81 then inserting a pin 90 of appropriate diameter intohole 89. The circumference of shaft 81 is then welded to bole 88 at 91,and pin 90 is finished flush with the external surface of hub 82.However, besides pinning and welding, other connecting means arecontemplated to practice the present invention such as mating male andfemale screw threads. Further details of Hub 82 using the same referencenumerals as in FIGS. 5 and 6 are shown in FIGS. 16-20.

[0032] Referring now to FIGS. 5-10, boss 92 has a proximal end 93, adistal end 94, and a body 95 located between ends 93 and 94. Proximalend 93 is fixedly secured to the distal end 86 of hub 82 in anyappropriate manner, such as being cast as a single piece, or machinedfrom a single piece of material; however, boss 92 and hub 82 arepreferably first pinned, then welded together. In this regard, proximalend 93 is placed in a mating slot 96 in hub 82 (FIGS. 16, 19, 20). Holes97 (FIGS. 5, 6) are then drilled through hub 82 and boss 92, which isthen followed by pinning (98) and welding the pins into place. Hereagain, as in the previously presented embodiment, hub 82 is preferablyslightly larger than boss 92 so as to from shoulder 100 between hub 82and boss 92. Body 95 has a length 170 (FIG. 10), and preferably length170 is such that device 80 will not overly penetrate the disc space whenshoulder 100 butts against the outside of the vertebrae. The distal end94 of boss 92 is closed and preferably gets thinner at tapered portion171 over a surface length 172 to a vertex 101. Similar to the groovescut in the boss of the previous embodiment, boss 92 is cut, preferablyon each side, with grooves 102, 103, and 104 (FIGS. 7 and 8), whichserve to engage tissue when the medical device is in use. Finally boss92 is also cut with slots 105 and 106, and channels 107 (FIGS. 7-9).Slots 105 and 106 are preferably cut completely through boss 92. Theseslots are used to hold a first longitudinal fin 83 and a secondlongitudinal fin 84. Channels 107 serve to inform the user where themidline of medical device 80 is located when the device is in use andplaced between two vertebrae.

[0033] Now referring to FIGS. 4-6, longitudinal fins 83 and 84 arepreferably identical to previously presented longitudinal fin 35,however, for the convenience of the reader, this information will bepresented again using different reference numerals for this embodimentof the present invention. Longitudinal fins 83 and 84 have a distal end108 and a proximal end 109 that preferably extends from boss 92 atvertex 101 and continues to extend from boss 92 for the entire surfacelength 172 of tapered portion 171. However, it is also contemplated thatlongitudinal fins 83 and 84 do not extend from surface length 171 forits entire dimension. That is from tapered portion's distal end atvertex 101 to its proximal end 199 (or simply does not extend from thesurface of the tapered portion at this location) for up to about 60percent of surface length 172. And preferably does not extend for up toabout the first, distal 60 percent of surface length 172. In otherwords, longitudinal fins 83 and 84 will extend from tapered portion 171for about 40 percent or more of length 172 and leave a remaining 60percent or less without a fin. But again, preferably longitudinal fins83 and 84 will extend from tapered portion 171 for about the first,proximal 40 percent or more of length 172 and leave a remaining distal60 percent or less without a fin. Examples of 60 percent, 20 percent,and 10 percent coterminous profiles are shown in previously presentedFIG. 4 at dashed lines 64, 65, and 66.

[0034] Longitudinal fins 83 and 84 are preferably mechanically held inslots 105 and 106 by pins 110 or a weld (not shown). The proximal ends109 of longitudinal fins 83 and 84 are preferably straight to rideagainst the bottom 111 (FIG. 8) of slots 105 and 106. The distal end108, however, preferably terminates at a second vertex 112. Longitudinalfins 83 and 84 each then preferably continue from vertex 112 to projectlaterally around distal end 94 of boss 92 and down at least a portion ofbody 95. Moreover, although longitudinal fins 83 and 84 are shown inFIG. 5 wrapping around both the top side 115 and the bottom side 116 ofboss 92, it is also contemplated that longitudinal fins 83 and 84 eachproject only from one of these sides, not both.

[0035] Referring now to FIGS. 5, 6, and 13-15, medical device 80preferably also includes aft guide 140 fixedly secured to shaft 81. Bothaft guide 140 and hub 82 preferably have a cross-section to slide withinan outer distractor sleeve. And here, the distractor would be onetypically used in a double-barrel procedure because, referring to FIG.15, the cross-section of aft guide 140 is approximately twice as wide asit is tall and resembles a figure “8”. In this embodiment, aft guide 140is first drilled at 144 to a diameter that is slightly larger than shaft81. Thereafter, aft guide 140 is slid over shaft 81 and located over thelength of shaft 81 at a desired location in line with hub 82 and boss92. Aft guide 140 is then first pinned at 141 and then welded at 142 and143 to secure it to shaft 81. Beyond pinning and welding, other methodsto attach aft guide 140 to shaft 81 are contemplated by this invention,for example, by casting aft guide 140 and shaft 81 from a common pieceof material.

[0036] Additional details of shaft 81 are further shown in FIGS. 11 and12. The area 150, which is proximal to aft guide 140, is preferablyknurled at 151 to assist the user in gripping the device. Moreover, theproximal end 152 of shaft 81 is further adapted at 153 to accept aremovable handle (not shown). In this embodiment, the adaptation 153comprises a male fitting with two flat sides 154 and 155. Acomplementary female fitting, which is attached to a handle, thenengages shaft 81 by sliding over the male fitting. The handle can thencontrol rotation of the shaft because the mating female fitting hascomplementary flat sides (not shown) that ride against mating flat sides104, 105 on end of shaft 81.

[0037] Each of the foregoing embodiments of the present invention isgenerally used in the same manner. With the spinal disc removed, thesurgeon places the leading edge of the longitudinal fins on the distalend of the distractor device into the disc space and against thesurrounding vertebrae 200, 201. (FIG. 21) Once properly located, thesurgeon then drives the distal end of the distractor device between thevertebrae and into the disc space 202. Upon entry, the longitudinal finsimmediately cut into the surrounding vertebrae, which helps prevent thedistractor device from deviating from the desired line of entry. Asurgeon typically drives the device into the disc space until shoulder55 or 100 comes to rest against the outside of vertebrae 200, 201. Oncefully inserted, the surgeon typically either removes the device andreinserts a taller device to further separate the vertebrae, or if thedesired separation has been reached, the surgeon installs a guidesleeve. With the guide sleeve in place, the surgeon then removes thedevice from between the vertebrae and continues the rest of theoperation through the guide sleeve.

[0038] Referring now to FIGS. 22-24, there is shown yet anotherembodiment of the present invention. For the convenience of the reader,like numerals have been used to identify similar parts of this inventionas that used in the embodiment shown in FIGS. 5-10. The embodiment shownin FIGS. 22-24, however, is for use when the surgeon must approach thespine from an angle. Here, medical distractor 80 has an oblique edge300, which is typically required for the surgeon to distract adjacentvertebral bodies from a non-perpendicular angle. Relevant here, obliqueangle 300 emphasizes the importance of the present invention.Longitudinal fin 83 helps insure that medical distractor 80 willmaintain the proper line of insertion, even though oblique angle 300 maytend to force distractor 80 to move laterally. Immediately upon contactwith the vertebral bodies, longitudinal fin 83 cuts into the vertebraeto help maintain the proper line of insertion into the disc space evenbefore oblique angle 300 has the opportunity to cause a lateralmisalignment.

[0039] An advantage of this invention over the prior art largely restsin the unique placement of the longitudinal fins 35 or 83-84 over asubstantial part of the tapered portion of the boss that supports thelongitudinal fin. Unlike the devices found in the prior art, the presentinvention places one or more guiding longitudinal fins in contact withvertebral bodies before they are fully distracted. This placement allowsthe longitudinal fins to help maintain the surgeon's line of insertionbefore the distractor device starts to separate the opposing vertebrae.And this assistance is far from minor. As explained earlier, thelocation of these “wedges” within the disc space can ultimatelydetermine the exact placement of spinal implants or fusion material.Hence, maintaining the proper line of insertion of a distractor deviceat the start is important to help assure that the surgery willultimately be a success.

What is claimed is:
 1. A tip for a medical device to distract twoopposing vertebral bodies, said tip comprising: (a) a hub with proximaland distal ends; (b) a boss, said boss fixedly secured to the distal endof said hub, said boss having a body, said body having a length; saidboss further having an external surface, and a tapered portion distal tosaid body, said tapered portion having a length; and said boss furtherhaving a first vertex distal to said tapered portion; and (c) a firstlongitudinal fin, said first longitudinal fin fixedly secured to saidboss, said first longitudinal fin projecting from at least a portion ofthe external surface of said boss, said first longitudinal fin extendingover at least a portion of the length of said body and substantiallyextending over the length of said tapered portion.
 2. The tip of claim1, where said first longitudinal fin is coterminous with said firstvertex.
 3. The tip of claim 1, where said first longitudinal finprojects distally of said first vertex.
 4. The tip of claim 1, includingmeans for connecting the proximal end of said hub to the distal end of amedical device.
 5. The tip of claim 1, where said first longitudinal finlaterally projects out from at least a portion of the body of said bosson two opposing sides of said boss.
 6. The tip of claim 1, where saidfirst longitudinal fin perpendicularly projects from the externalsurface of said boss.
 7. The tip of claim 1, where said firstlongitudinal fin includes a second vertex projecting distally of saidfirst vertex of said boss.
 8. The tip of claim 1, where said hub extendslaterally beyond said boss thereby forming a shoulder between said huband said boss.
 9. The tip of claim 1, including a second longitudinalfin fixedly secured to said boss, said second longitudinal finprojecting from at least a portion of the external surface of said boss,said second longitudinal fin extending over at least a portion of thelength of said body and substantially extending over the length of saidtapered portion, and said second longitudinal fin being locatedsubstantially parallel to said first longitudinal fin.
 10. The tip ofclaim 9, where said first and second longitudinal fins are coterminouswith said first vertex.
 11. The tip of claim 9, where said first andsecond longitudinal fins project distally of said first vertex of saidboss.
 12. The tip of claim 9, where said first and second longitudinalfins laterally project down at least a portion of the body of said bosson two opposing sides of said boss.
 13. A medical device for distractingtwo opposing vertebral bodies, said medical device comprising: (a) ashaft, said shaft having a distal end; (b) a hub with proximal anddistal ends, the proximal end of said hub being fixedly secured to thedistal end of said shaft; (c) a boss, said boss fixedly secured to thedistal end of said hub, said boss having a body, said body having alength; said boss further having an external surface, and a taperedportion distal to said body, said tapered portion having a length; andsaid boss further having a first vertex distal to said tapered portion;and (d) a first longitudinal fin, said first longitudinal fin fixedlysecured to said boss, said first longitudinal fin projecting from atleast a portion of the external surface of said boss, said firstlongitudinal fin extending over at least a portion of the length of saidbody and substantially extending over the length of said taperedportion.
 14. The medical device of claim 13, where said firstlongitudinal fin is coterminous with said first vertex.
 15. The medicaldevice of claim 13, where said first longitudinal fin projects distallyof said first vertex.
 16. The medical device of claim 13, where saidfirst longitudinal fin laterally projects out from at least a portion ofthe body of said boss on two opposing sides of said boss.
 17. Themedical device of claim 13, where said first longitudinal finperpendicularly projects from the external surface of said boss.
 18. Themedical device of claim 13, where said first longitudinal fin includes asecond vertex projecting distally of said first vertex of said boss. 19.The medical device of claim 13, where said hub extends laterally beyondsaid boss thereby forming a shoulder between said hub and said boss. 20.The medical device of claim 13, including a second longitudinal finfixedly secured to said boss, said second longitudinal fin projectingfrom at least a portion of the external surface of said boss, saidsecond longitudinal fin extending over at least a portion of the lengthof said body and substantially extending over the length of said taperedportion, and said second longitudinal fin being located substantiallyparallel to said first longitudinal fin.
 21. The medical device of claim20, where said first and second longitudinal fins are coterminous withsaid first vertex.
 22. The medical device of claim 20, where said firstand second longitudinal fins project distally of said first vertex ofsaid boss.